Work conducted in collaboration with Jerry Nepom and I.Hellstrom has shown that a specific blocking factor (SBF) which has a molecular weight of 56,000 daltons can be isolated from the sera of mice carrying transplanted chemically induced tumors and is responsible for the ability of such sera to block specific cell-mediated immunity to tumor antigens in vitro. The blocking effect shows the same specificity as the tumor specific transplantation antigens (TSTA) of the tumor borne by the serum donor. Our next approach was to study whether a similar SBF can be isolated from a growing tumor. Tumor tissue fluids were obtained by pressing tisssues from chemically induced mouse sarcomas through stainless steel screens into culture medium followed by centrifugation. After that a toxic effect of concentrated such fluids had been diluted out, a blocking activity remained which showed the same specificity as the TSTA of the given tumor. When further studied by immunoadsorption procedures blocking factors isolated from tumors were found to have characteristics similar to those of SBF isolated from serum. Experiments were then performed to test whether blocking material isolated directly from tumors could enhance tumor growth in vivo. Two effects were seen. First, it was shown that tumor tissue fluid prepared as described above could enhance the growth of tumors having the same antigenic specificity as well as of tumors having different tumor specific transplantation antigens. Second, a tumor specific enhancing effect was also demonstrated when fluid containing the "right" as compared to the "wrong" tumor specific transplantation antigens were inoculated; this was particularly seen with the more diluted fluids. A modified binding assay has been introduced by which radiolabelled protein A from Staphylococcus aureus is used to demonstrate the binding of antibodies to surfaces of tumor cells plated in vitro. This assay is highly sensitive and very reproducible. It can demonstrate antibodies to H-2 antigens up to serum dilutions of around 1:100000. It can also demonstrate the presence on tumor cells of certain MuLV antigens.